Abstract

The world-class Loulo mining district (15.5 Moz resource), in the Birimian terrane of western Mali, contains a range of mineralogically diverse styles of orogenic gold mineralization. The district is distinguished by As-rich orebodies as at Yalea, as well as tourmaline-bearing, Fe-rich, orebodies as at Gara. New fluid inclusion and stable isotope data presented here constrain the nature of the hydrothermal fluids responsible for these different types of mineralization, and point towards the role of multifluid sources (metamorphic and magmatic) in the formation of orogenic gold deposits.

Microthermometric and laser Raman studies from Yalea Main and two other similar orebodies (Loulo-3 and Baboto) reveal the dominance of coexisting CO2-N2±CH4 (type 1) and H2O-NaCl (type 2) fluid inclusions. These inclusions show evidence of fluid unmixing from reduced (quartz-faylite-magnetite, QFM, buffered), low-salinity (≤10 wt % NaCl equiv), CO2-rich-H2O-NaCl-N2 ± CH4 primary ore fluids. The combination of microthermometric data and geothermometry based on ore and alteration assemblages indicate trapping temperatures and pressures of 270° to 340°C at 1.4 to 1.8 kbar. The P-T-X state of the Yalea-style mineralizing fluids, along with the δ18Ofluid compositions of 8.8 to 10.7‰, is consistent with the derivation of auriferous fluids during greenschist facies regional metamorphism of the host terrane. Similar fluid compositions were previously reported elsewhere in the Birimian crust of West Africa and in other orogenic gold districts worldwide. The precipitation of gold from the H2S-rich metamorphic fluid is primarily linked to phase separation of the ore fluid, which is controlled by P-T fluctuations and/or fluid-rock interaction with carbonaceous host sedimentary rocks (confirmed by low δ13C values of −21.7 to −15.8‰).

Fluid inclusion investigations from Gara and a similar style orebody (Yalea North) indicate the presence of coexisting CO2 ± N2 ± CH4 (type 1) and mixed-salinity (5–21 wt % NaCl equiv) CO2-rich-H2O-NaCl inclusions (type 3). Inclusion assemblages also contain common oxidized (hematite-magnetite, HM, buffered), high temperature (>400°C), hypersaline (∼35–50 wt % total dissolved solids), metalliferous (Na+Fe+Ca+Cu+Ni+W+ Pb+Zn), multiphase H2O-rich-CO2-NaCl-FeCl2 inclusions (type 4). This inclusion type has not been previously reported in other Birimian terranes. The composition of the brines, along with carbon isotope data (δ13C of −14.4 to −4.5‰), suggests a magmatic input to the Gara-style hydrothermal system. The coexistence in the fluid inclusion assemblages of the magmatic brines with Yalea-style, CO2-rich, metamorphic fluids and the positive correlation between salinity and homogenization temperatures suggest mineralization was locally controlled by fluid mixing. The interaction of these two chemically contrasting fluids explains the distinctive petrographic characteristics of the Gara-style orebodies. This includes the growth of widespread multi-stage Fe3+-rich tourmaline (B-rich granite source) and sodic alteration, and ore assemblages consisting of abundant nickeloan pyrite, (REE)-phosphates, Ni ± Co ± Pb ± Zn minor/trace sulfides, and scheelite. Gold deposition in the Gara-style hydrothermal system is related to physical and chemical changes of the two fluids during mixing (e.g., decreases in fO2 and T in the brines and retrograde boiling of the CO2 component in the metamorphic fluids, a “salting out effect”).

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